Evolved High-Speed Packet Access (HSPA) is a mobile data protocol defined in the 3rd Generation Partnership Project (3GPP) release 7. Evolved HSPA includes increased data speed on the downlink (DL) from a base station (BS) and on the uplink (UL) to a BS, lower state transition latency, and longer battery life for mobile terminals. Evolved HSPA (HSPA Evolution) provides data rates up to 42 Mbit/s on the DL with multiple input multiple output (MIMO) technologies and 11.52 Mbit/s on the UL with higher order modulation.
The evolution of HSPA towards a higher throughput and lower latencies requires improvements to the physical layer, as well as the architecture. One improvement is the use of a Dual Cell-High Speed Downlink Packet Access (DC-HSDPA) to increase the DL capacity and to support hot spot activities.
DC-HSDPA introduces a second DL carrier to be used as a second high speed downlink packet access (HSDPA) channel. In DC-HSDPA, the dual cell operation applies to the high-speed downlink shared channel (HS-DSCH). In addition, the dual cells of DC-HSDPA are associated with a single BS and operate on different carriers. For example, one cell operates on an anchor carrier and the other cell operates on a supplementary carrier where both carriers are transmitted by the same BS. Further, in Release 8 of the HSPA specifications, the two cells of DC-HSDPA operate with a single transmit antenna and the two cells operate over adjacent carriers in the same frequency band, although in future releases the same band restriction may be removed.
In such a dual-cell HSDPA network, BSs, referred to as Node-Bs in 3GPP, communicate to mobile terminals or other wireless transmit/receive units (WTRUs) over two distinct carriers simultaneously. This results in doubling the bandwidth, doubling the peak data rate available to WTRUs. It also has the potential to increase the network efficiency by means of fast scheduling and fast channel feedback over two carriers.
As with conventional wireless apparatus, network stations and WTRUs for Dual-Cell HSDPA communications are configured with multi-layer communication processing components that implement a first physical layer (L1) that transmits and receives the wireless signals, a medium access control (MAC) layer (L2) and various higher layers.
In Dual-Cell communications, each WTRU is assigned a so-called anchor carrier. The anchor carrier for the WTRU carries dedicated and shared DL control channels, such as, for example, a fractional dedicated physical channel (F-DPCH), an enhanced dedicated channel (E-DCH) hybrid automatic repeat-request (ARQ) indicator channel (E-HICH), an E-DCH relative grant channel (E-RGCH), an E-DCH absolute grant channel (E-AGCH), a common pilot channel (CPICH), etc. In addition anchor carrier for the WTRU carries data channels, such as a HS-DSCH. The optional supplementary or secondary carrier that serves the HS-DSCH cell carries data channels and a CPICH for the WTRU. The anchor carrier for a given WTRU may correspond to the supplementary carrier for another WTRU.
FIG. 1 shows an example evolved HSPA communications 100 using a DC-HSDPA setup. Two DL carriers are shown, an anchor carrier A and a supplementary carrier B transmitted by a base station 101 to a WTRU 102 over the same geographic area, along with a single UL carrier A transmitted by the WTRU. The UL carrier A is associated with the anchor DL carrier A and also provides feedback associated with the second supplement DL carrier B.
Although only a single WTRU is illustrated, the DL carriers may be concurrently utilized for wireless communications with other WTRUs. As noted above the supplementary DL carrier B for WTRU 102, may be being used as an anchor carrier for another WTRU.
During low utilization periods of DC-HSDPA, it is possible that the second carrier may not be used, potentially underutilizing resources. This provides an opportunity to use the second carrier transmission as a diversity channel when it is not fully utilized for DL transmissions.
The typical number of HARQ processes used for wireless communication is sixteen. With Dual-Cell HSDPA, there are two alternatives for associating each carrier to hybrid ARQ (HARQ) entities: (1) Using separate HARQ entities, each HARQ entity is assigned to a given carrier (e.g., 8 HARQ processes per carrier) and (2) using joint HARQ entities, the HARQ entities are not linked to a specific carrier (e.g., all 16 HARQ processes are available to both carriers). In the first alternative, the HARQ retransmissions can only occur over the same carrier. In the second alternative, the HARQ retransmissions can be transmitted over a different carrier, if desired.
FIG. 2 illustrates an example of a Universal Terrestrial Radio Access Network (UTRAN) side MAC-enhanced high speed (MAC-ehs) architecture for DC-DSDPA with separate HARQ entities. FIG. 3 illustrate an example of a UTRAN side MAC-ehs architecture for DC-HSDPA with a joint HARQ entity.
Using separate HARQ entities has some implementation advantages. By assigning each carrier to a HARQ entity, no change is required in Layer 1 (L1) specifications and the current high speed shared control channel (HS-SCCH) type 1 can be used without modification. However, doing so reduces the fast scheduling gain, i.e. the gain obtained by exploiting the fast channel variations independently over the two carrier frequency, since retransmissions are then constrained to occur on the same carrier as the initial transmission. Also, in cases where the supplementary carrier is disabled, any on-going HARQ transmission on the supplementary carrier would be blocked. This implies that either the corresponding HARQ entities have to be flushed, resulting in loss of data and additional transmission delays, or that a new more complex procedure needs to be devised to avoid losing data.
When using joint HARQ entities on the other hand, the scheduler can take full advantage of the varying radio conditions and, in addition, all active HARQ processes can be maintained when de-activating the supplementary carrier. Therefore, there are significant advantages to using joint HARQ entities.
While using joint HARQ entities is advantageous, it requires additional signaling. The existing HS-SCCH type 1 only carries three bits for HARQ process information. This is insufficient to address the 16 HARQ processes that are typically available with joint HARQ entities.
Under Long Term Evolution-Advanced (LTE-A) standards, carrier aggregation and support of flexible bandwidth arrangement may be supported. This allows DL and UL transmission bandwidths to exceed 20 MHz. In Release 8 (R8) LTE, for example, a 40 MHz bandwidth is specified. This improvement will also allow for more flexible usage of the available paired spectrum. For example, R8 LTE is limited to operate in symmetrical and paired FDD mode, e.g., DL and UL must have the same transmission bandwidth, e.g., 10 MHz, 20 MHz, and so on. However, LTE-A should be able to support operation in asymmetric configurations such as a 10 MHz DL carrier paired with a 5 MHz UL carrier. In addition, composite aggregate transmission bandwidths should also possible with LTE-A, e.g., a first 20 MHz DL carrier and a second 10 MHz DL carrier paired with a 20 MHz UL carrier and so on. Additionally, composite aggregate transmission bandwidths may not necessarily be contiguous in frequency domain, e.g., the first 10 MHz so-called component carrier in the above example could be spaced by 22.5 MHz in the DL band from the second 5 MHz DL component carrier. Alternatively, operation in contiguous aggregate transmission bandwidths should also be possible, e.g., a first 15 MHz DL component carrier is aggregated with another 15 MHz DL component carrier and paired with a 20 MHz UL carrier.
In a LTE-A system, the physical downlink control channels (PDCCHs) or Downlink Control Information (DCI) messages contained therein carrying the assignment information can be separately transmitted for the component carriers containing the accompanying physical downlink shared channel (PDSCH) transmissions. For example, if there are two component carriers, there are two separate DCI messages on each component carrier corresponding to the PDSCH transmissions on each component carrier respectively. This is referred as separate PDCCH coding.
Alternatively, the two separate DCI messages for a WTRU may be sent on one component carrier, even though they may pertain to accompanying data, or PDSCH transmissions on different component carriers. The separate DCI messages of PDCCHs for a WTRU or a group of WTRUs can be transmitted in one or in multiple carriers, and not necessarily all of them on every component carrier. This is referred to as an anchor carrier with separate PDCCH coding.
The DCI carrying the assignment information for PDSCH(s) on more than one component carrier can be encoded jointly and carried by one single joint DCI control message, or PDCCH message. This is referred to as joint PDCCH coding. One approach for the dynamic support of multicarrier assignment schedules is to have a variable size DCI format with a common DCI part and a carrier specific part. The common DCI part contains a field indicating which component carrier is being assigned in the current subframe and implicitly which carrier specific DCI format will follow. The common DCI format may also contains information that could apply to all carriers such as Hybrid ARQ process.
Accordingly, a method and apparatus configured to provide the necessary mechanisms to signal the HARQ process information in a DC-HSDPA system or LTE-A system with joint HARQ entities and to effectively utilize the unused resources in supplementary component carriers to expand system performance and reliability is desired.